PURPOSE: To validate a threshold-based prototype software application (MeVis PULMO 3D) for quantification of chronic interstitial lung disease (ILD) in patients with rheumatoid arthritis (RA) using variable threshold settings for segmentation of diseased lung areas. METHODS: Twenty-two patients with rheumatoid arthritis were included and underwent thin-section CT (4x1.25mm collimation). CT scans were assessed by two observers for extent of ILD (EoILD), and twice by MeVis PULMO 3D for each protocol. MeVis PULMO 3D used four segmentation threshold (ST) settings (ST=-740, -780, -800 and -840HU). Pulmonary function tests were obtained in all patients. Statistical evaluation used 95% limits of agreement (LoA) and linear regression analysis. RESULTS: There was total concordance between the software measurements. Interobserver agreement was good (LoA=-28.36 to 17.58%). EoILD by readers correlated strongly with DL(CO) (r=-0.702, p<0.0001) and moderately with FVC (r=-0.523, p=0.018). There was close correlation between readers and MeVis PULMO 3D with best results for ST <780HU (EoILD vs. MeVis PULMO 3D: r=0.650 for ST=-800 and -840HU, respectively; p=0.002). MeVis PULMO 3D correlated best with DL(CO) at ST of -800HU (r=-0.44, -0.49, -0.58 and -0.57 for ST=-740, -780, -800 and -840, respectively; p=0.007-0.05) and moderately with FVC (r=-0.44, -0.51, -0.59 and -0.45 for ST=-740, -780, -800 and -840), respectively; p=0.007-0.05). CONCLUSION: The MeVis PULMO 3D system used holds promise to become a valuable instrument for quantification of chronic ILD in patients with RA when using the threshold value of -800HU, with evidence of the closest correlations, both with human observers and physiologic impairment.